Semiconductor devices and other microelectronic devices are typically manufactured on a wafer or workpiece having a large number of individual dies (e.g., chips). Each wafer undergoes several different procedures to construct the switches, capacitors, conductive interconnects, and other components of a device. For example, a wafer may be processed using lithography, implanting, etching, deposition, planarization, annealing, and other procedures that are repeated to construct a high density of features. One aspect of manufacturing microelectronic devices is evaluating the wafers to ensure that the microstructures meet desired specifications.
Scatterometry is one technique for evaluating several parameters of microstructures. With respect to semiconductor devices, scatterometry may be used to evaluate film thickness, line spacing, trench depth, trench width, and other aspects of microstructures. Many semiconductor wafers, for example, include gratings in scribe lanes between the individual dies to provide a periodic structure that can be evaluated using scatterometry equipment. One scatterometry process includes illuminating these periodic structures on a wafer and obtaining a representation of the scattered radiation returning from the periodic structures. The representation of return radiation, or the structure's “diffraction signature,” is then analyzed to estimate one or more parameters of the microstructure.
One challenge of scatterometry is determining the orientation of a grating on a wafer during the scatterometry measurement process. The grating orientation is important when comparing measured scatterometry image data to model image data because it affects the diffraction signature of the grating. The grating orientation on a measured wafer must be essentially equivalent to a simulated orientation found in modeled wafer data to produce diffraction signatures that are meaningfully comparable, and to therefore determine whether the wafer meets desired specifications. Thus, if the grating orientation on the measured wafer does not match the simulated or actual grating orientation in the model image data, the diffraction signature of the measured image data cannot properly be compared to that of the model image data without first adjusting the measured image data.
Another challenge of scatterometry involves measurements on gratings where the orientation of the grating structure is not well-controlled. In the case where variability exists in the orientation of the grating due to process or manufacturing variations, this variability will affect the scatterometry measurement because the orientation of the measured grating will not match the orientation of the simulated grating, and the measured scattered data will therefore be inconsistent.
Many existing scatterometry systems include a Cartesian coordinate stage or “x-y-θ” stage for moving a wafer during measuring. In these systems, wafer movement is generally limited to three degrees of freedom within an x-y plane during measuring, namely lateral, longitudinal, and rotational movement. Some Cartesian coordinate stages may also be movable vertically, and may therefore be referred to as x-y-z-θ stages. During the scatterometry measuring process, an x-y-θ stage is generally moved within the x-y plane under the optics head of the scatterometer until all of the desired regions of the wafer are measured.
A pre-alignment tool is commonly used in scatterometry systems employing an x-y-θ stage to properly align the wafer grating relative to the scatterometer optics. For example, upon loading of the wafer into the scatterometry system, a pre-alignment tool may align the wafer grating using a notch on the wafer, which has a known orientation with respect to the grating. The wafer may then be slightly rotated, if necessary, to achieve the exact desired grating orientation relative to the optics of the scatterometry device, i.e., to align the grating such that the measured image data is comparable to the model image data to determine whether the wafer meets desired specifications. Pre-alignment systems are generally imperfect, however, and may introduce unwanted rotation of the grating orientation due to alignment errors.
Another challenge in scatterometry is introduced when a polar coordinate stage, or an “x-θ” stage, is used instead of on an x-y-θ stage. In these systems, the x-θ stage is movable in a longitudinal direction during measuring, while the wafer rotates on the x-θ stage. While this presents an advantage because any point on the wafer can be accessed using just two stages of movement, the orientation of the grating structure when it arrives beneath the imaging optic will vary depending on its location on the wafer. To compensate for this, the optical system must be rotated, which is difficult and time-consuming, or the model simulations must be performed to be consistent with the grating orientation, which is also time-consuming. As a result, x-θ polar coordinate stages have not been very useful for making typical scatterometry measurements.